Medical device for generating a therapeutic parameter

ABSTRACT

A device for generating a therapeutic value for a patient as a function oft least one variable parameter picked up within the body and constituting a first input value, with a change in the first parameter being a function of a second parameter which also constitutes an input value. The device includes circuitry for varying the generation of the therapeutic value by varying the second parameter so that the difference of the values of the first parameter, at selected limits of a variation range of the first parameter, constitutes a maximum in an intended treatment range of the patient. A memory retains a value of the second parameter for which the variation range of the first parameter constitutes a maximum. Control circuitry changes the therapeutic value as a function of the first parameter while maintaining the previously stored second parameter.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The invention relates to a medical device of the type for generating atherapeutic value for a patient as a function of at least one variableparameter picked up within the body and constituting a first inputvalue, with a change in the first parameter being a function of a secondparameter which also constitutes an input value.

2. Background Information

In such devices which generate a therapeutic value for a patient from avalue (parameter) picked up within the patient's body, there oftenexists the problem that this parameter, with the aid of which thepatient is treated, is a function of another parameter about which noreliable knowledge is available in connection with the intendedtreatment to enable it to be used as a further variable in the course ofthe treatment or which cannot be considered for other reasons. Since thefurther parameter thus takes on a relatively random value in each case,success of the treatment cannot be ensured with the necessaryreliability.

Thus, it has not yet been possible in connection with cardiac pacemakersto provide a reliable rule for influencing the stimulation rate withmaximum efficiency as a function of conductivity values that were pickedup in the heart within the cardiac cycle at certain points in timeduring the pre-ejection period.

The obtained values differed greatly from patient to patient so that thetransfer of settings found for one patient to another patient wasincreasingly connected with difficulties.

Usually it was the increase in conductivity that was evaluated,determined by electrodes installed in the right ventricle whichpreferably may simultaneously constitute electrodes of the stimulationsystem.

SUMMARY OF THE INVENTION

It is the object of the invention to make it possible, in a medicaldevice of the above-mentioned type, to influence in such a way theacquisition conditions for the parameter that controls a therapeuticvalue and is picked up within the patient's body that the sensitivity ofthe control or regulation becomes a maximum and, in particular, theregulation algorithms to be employed for various patients can betransferred to other patients.

This is accomplished by means for varying the generation of thetherapeutic value by varying the second parameter so that the differenceof the values of the first parameter, at selected limits of a variationrange of the first parameter, constitutes a maximum in an intendedtreatment range of the patient, a memory in which a value of the secondparameter for which the variation range of the parameter constitutes amaximum is retained; and a control means for changing the therapeuticvalue as a function of the first parameter while maintaining thepreviously stored second parameter.

The invention is based on the realization that often there are one orseveral further parameters which influence the acquisition of theparameter to be evaluated in an undetected manner. If it is now possibleto set this parameter for the measurement in such a way that the changein the parameter to be evaluated becomes a maximum in the intendedtreatment range for the patient, the information available for a changein the value influencing the treatment of the patient can in many casesbe significantly improved or discovered at all.

Thus, a further parameter in the form of another value that influencesthe patient is changed by means of suitable measures in such a way thatthe influencing of physical processes within the patient is maximized asa function of a first value derived from the body so that maximumresponse to the treatment is ensured. This maximization of theinfluenceability of the patient is determined during a "learning cycle"of the device and the determined result is then used for the furthertreatment.

In the medical device of this type for generating a therapeutic valuefor a patient as a function of at least one first parameter picked upwithin the body it is initially assumed that the first parameter is afunction of a further (second) parameter. This may be, for example, a(possibly temporary) masking of the signal responsible for a change inthe therapeutic value to be picked up within the patient's body (firstparameter).

If the first parameter can be positively changed by means of externalmeasures so that it passes through the variation range which issignificant for the patient's treatment, if thus, all conditions, forexample, all conditions in the patient's daily life under whichtreatment is to take place--particularly by means of portable orimplantable medical devices--can be simulated or set, it is possible toobserve the corresponding change in the first parameter.

The measures according to the invention now generate the therapeuticvalue by varying the second monitorable parameter in such a way that thedifference of the values of the first parameter at the limits of thevariation range of the first parameter constitutes a maximum.

A preferable further parameter to be changed during signal pickup withinthe patient's body is a time window during which the signals are pickedup within a particular cyclic sequence, such as the cardiac rhythm oranother biorhythmic cycle. Another variable parameter is possibly givenby the location where the signal is obtained--for example, if it ispossible to switch between several sensors distributed in space, such aselectrodes in a conductivity value determination.

This setting of the second controllable parameter is now stored in amemory and used as a basis for further influencing the therapeutic valuefor the patient as a function of the first parameter so that the controlof the therapeutic value within the variation range of the firstparameter takes place while maintaining the thus obtained secondparameter.

As another advantageous feature of the invention, different secondparameters are determined for different variation ranges of the firstparameter, each associated with such a variation range, and are storedin the memory. Now--even if the influence of the second parameter on thedependency of the value influencing the patient upon the first parameterdoes not remain unchanged over the variation range of the firstparameter, appropriate switching of the selection of the secondparameter from section to section permits an optimization of the controlor regulation of the value that influences the patient. The switching ofthe influence of the second parameter may here be effected by way ofadditional means; it may be changed by a third parameter which changesin its tendency in the same manner as the first parameter and can alsobe derived from within the patient's body or from his environment.

In another preferred embodiment, the switching of the selection of thesecond parameter may also be effected by the first parameter itself ifcare is taken by way of a switching hysteresis or other suitablemeasures that the control has the necessary stability and hunting cannotoccur.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantageous features of the invention are defined in thedependent claims and will now be described in greater detail inconnection with a description of the preferred embodiment of theinvention and reference to the drawing figures, in which:

FIG. 1 depicts a cardiac cycle signal and a time window in which thesignals are picked up;

FIG. 2 depicts an embodiment of a circuit for the requirement-dependentchange of the stimulation rate of an artificial cardiac pacemaker; and

FIG. 3 depicts a further embodiment of the circuit according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention is advantageously employed with implantable cardiacpacemakers that are provided with a circuit for therequirement-dependent change of the stimulation rate. The electricalconductivity K measured in the heart by means of a stimulation electrodeE₁ is shown in FIG. 1 as a cardiac cycle with time window.

Particularly the change in electrical conductivity in the heart, as thefirst parameter, constitutes a measure for physical stress that requiresa corresponding pumping output (volume per minute) of the heart.Regardless of whether the circuit involved is a closed regulatingcircuit or a control circuit, this value is an input value for therequirement-dependent circuit for changing stimulation rate 10. Thestimulation rate of the cardiac pacemaker (for on-demand pacemakers, thebasic rate) is varied by a control and/or regulating device 3 as afunction of the value representing physical stress.

This representative value is the increase S in electrical conductivity κpreferably in the right ventricle, that is, the difference between theelectrical conductivity Δκ at the beginning and at the end of a certainpredeterminable time interval Δt_(ij).

To this end, a time window Z_(ij) =f(Δt_(i), Δt_(j)) is provided by amemory 6 and constitutes the second parameter which influences themeasuring signal. In an input circuit 1, a fixed time reference pointt_(O) is derived from the input value. Thus, Δt_(ij) is thepreselectable time interval that begins at time t_(i), that is, at adistance Δt_(i) from reference point t₀, and ends at a distance Δt_(j)from reference point t_(O). In the past, many unsuccessful attempts havebeen made to find a respectively suitable time window which yieldsreproducible results with sufficient accuracy and, in particular, alsopermits a transfer to other patients.

The change (increase) in the short-term integral of the electricalconductivity within a certain time window within the cardiac cycle thusconstitutes at least indirectly an input value for the circuit for therequirement-dependent change of the stimulation rate 10.

According to an advantageous feature of the invention, that timeinterval (time window) is now determined and utilized for the laterdifference formation (determination of increase) of the measuredconductivity values in which a difference for two different stressstates of the patient constitutes a maximum. Another variation is that aswitch is made between electrodes arranged differently in space until asignal is found that has the greatest significance for the respectiveparameter as a function of which a value within the patient's body is tobe influenced.

For the first embodiment of an implantable cardiac pacemaker, FIG. 2shows a circuit for the requirement-dependent change of the stimulationrate 10. The electrical conductivity κ measured within the heart bymeans of the stimulation electrode E₁ is processed in a first inputcircuit 1 and in a second input circuit 2, and constitutes an inputvalue for a control and/or regulating device 3 in the circuit forrequirement-dependent changes in the stimulation rate 10.

The amplified conductivity signal determined in the right ventricle byway of suitable pickup points at the stimulation electrode E1, andtransmitted from the first input circuit, to the second input circuit 2,is integrated in the time interval in the second input circuit 2 inorder to eliminate short-term fluctuations that might falsify themeasurement signal.

By way of an analog-digital converter 7 and input circuit 1, inputcircuit 2 is connected with control and/or regulating device 3,particularly with its input/output channel 14. In addition toinput/output channel 15, the control and/or regulating device includes alogic unit, preferably a processor 11, and selection circuits 4, 5, 9suitable for connection of a memory, as well as a data input/outputcircuit 15 for the increase limit values, a rate selection circuit 8 forcontrolling the variation ranges, and a clock pulse generator 16.

The control and/or regulating circuit 3 is connected, on by way of itstwo selection circuits 4 and 9 for selecting the time intervals with thesecond input circuit 2, with a memory region selection circuit 5 withmemory 6. As a further feature, the data input/output circuit 15 is alsoconnected with memory 6 in order to store limit values for the firstparameter. The input/output, the input/output circuit 14 of controland/or regulating circuit 3 is further connected with a stimulationsignal output circuit 17.

The circuit for the requirement-dependent change of the stimulation rate10 further includes a communication unit 18 that is coupled with thecontrol and/or regulation circuit 3. In addition to programming theimplanted cardiac pacemaker, the communication unit 18 serves totransmit data to external programming and monitoring units.

Controlled by processor 11, a memory region is selected by way of thememory region selection circuit 5 which is connected with memory 6. Nowthe particular time interval (time window) at which the difference fortwo different stress states of the patient constitutes a maximum.Controlled by processor 11, the data in memory 6 are selected by way ofmemory selection circuit 5.

For this purpose it is initially necessary to pick up a series ofmeasurements for different physical stresses during which the respectivetime window is varied. To this end, control and/or regulating device 3is provided with a selection circuit 9 for selecting the time windowdistance Δt_(i) from reference point t₀. Selection circuits 4, 5 and 9include either independent forward/backward counters that are actuatedby the control and regulating device 3, or they are alternatelyadvantageously realized as software in processor 11 which, inparticular, evaluates the measured values and automatically finds theregions of maximum increase Δκ_(max) for a fixed setting (storage inmemory 6).

First, during the lowest stress stage B₀, the associated first parameterΔκ_(0ij) is successively determined for different time windows in theindividual cardiac cycles, with the length of time intervals Δt_(i) andΔt_(j) being varied. In stress stage B₁, a first time window withassociated first parameter Δκ_(1ij) is stored in memory 6. If duringthis stress stage a time window occurs that includes time intervalsΔt_(m) and Δt₁ as well as an associated first parameter to which theinequality Δκ_(1ml) -Δκ_(0ml) >Δκ_(1ij) -Δκ_(oij) applies, this timewindow Z_(ij) is overwritten by the new time window Z_(m1) which has thenew highest absolute value of the differences Δκ_(1ml) -Δκ_(0ml) of thefirst parameters. At the same time, the value Δκ_(1ml) associated withstress stage B₁ is stored in memory 6.

A stress stage B₀ has thus precisely one time window Z_(ij) withassociated time intervals Δt_(i) +Δt_(j). Thus, particularly with theuse of a processor 11 that is coupled with memory region selectioncircuit 5, the time window at which the difference in increase of theintegral of the conductivity values constitute a maximum is retained inits position relative to a fixed reference point in the cardiac cycle(limits of the pre-ejection period) for at least one further stressstage B₁ of the stress range.

The absolute values of the increases are here of importance which, inthe range involved, may of course also change their sign. It is alsoadvantage that, in the curve shown in FIG. 1, only those regions of areconsidered where the increase is monotonous.

A further embodiment of the invention will be described in greaterdetail with reference to FIG. 3. FIG. 3 depicts an embodiment of thecircuit for the requirement-dependent change of the stimulation rate 10in which the determination of the reference time t₀ and the formation ofthe short-term integral are performed by logic unit/processor 11 itself.Processor 11 is connected with memory 6 by way of an interface circuit19. The electrical conductivity κ measured in the heart by means ofstimulation electrode E₁ is amplified and fed by way of an input circuit1 to analog/digital converter 7 which is connected with input/outputchannel 14. An integrator 2 (as in the FIG. 2) and the associatedconnections are not required since the integration is effected bysoftware and processer 11. The determined increase S in the electricalconductivity κ, preferably in the right ventricle, that is, thedifference between electrical conductivity Δκ_(ij) at the beginning andend of a certain predeterminable time interval Δt _(ij) causes rateselection circuit 8 to vary the stimulation rate of the cardiacpacemaker.

In order to better approximate the integral by way of parabolas,processor 11 samples an odd number of measuring points at uniform timeintervals within time window Z_(ij) =f(Δt_(i), Δt_(j)) and inputs themeasured values through analog/digital converter 7 and input/outputchannel 14.

The different stress states constituting the reference values are, onthe one hand, preferably a first state of low physical stress B₀ (reststate) and, on the other hand, a second state in the range of arelatively great physical stress B₁ (working state). The mathematicalevaluation of the differences of the integrals is preferably effectedaccording to the Simpson theorem so that the calculating efforts of aprocessor provided in the pacemaker circuit are greatly reduced.

The entire range traversed during changes of stress may also besubdivided into a number of successive stress zone ranges which haveassociated stress stages. For the n^(th) stress stage B_(n), preciselyone time window Z_(ij) with time intervals Δt_(i) and Δt_(j) is thendetermined and care is taken that switchable intermediate ranges areavailable for different stress zone ranges.

Switching the influence of the second parameter on the first parameteris here done, for example, by means of an additionally provided activitysensor 12 which detects the intensity of the movements or other physicalactivity. Activity sensor 12 is connected with processor 11 by way of athird input circuit 13 of control and/or regulating device 3. By way ofdata input/output circuit 15 (FIG. 2), for examples, processor 11controls the storage for the respective stress stage and the call-up ofeither a value Δκ_(max) if the lower conductivity limit κ_(i) is fixed,or storage of the limit values κ_(i) and κ_(j) into or from memory 6,respectively. According to the limit values, rate selection circuit 8determines the stimulation rate. Using processor 11, the function ofrate selection circuit 8 may also be realized as software.

Such an activity sensor 12 may then also serve as self-calibration ormonitor in that it determines which changes in conductivity are to beassociated with certain stimulation rates.

In another embodiment which does not include the third input circuit,the switching of the influence of the second parameter on the firstparameter is effected by the output signal of the circuit for therequirement-dependent change of stimulation rate 10 itself. If necessarya switch is made with hysteresis whenever the processor determines anincrease (Δκ_(max) in the time window) to which another stress stageB_(n+l) can already be associated. The prerequisite for this is that thesystem must be self-adapting, that is, an attempt is made during thestimulation to find regions with a greater change in increase.

The circuit technology for a rate controlled pacemaker can be stipulatedto be known, with it being important in this modification of the presentinvention that the signal pickup of the parameter determining thestimulation rate is improved. In this connection, the use of theinvention is not dependent on whether the control is an open loopcontrol or a closed loop control.

The present invention is not limited in its embodiments to theabove-described preferred embodiments. Rather, a number of variationsare conceivable which take advantage of the described solution even forbasically different configurations.

We claim:
 1. A medical device for generating a therapeutic value for apatient as a function of at least one variable first parameter picked upwithin the body and constituting a first input value, with a change inthe first parameter being a function of a second parameter which alsoconstitutes an input value, said device comprising:means for varyinggeneration of the therapeutic value by varying the second parameter sothat a variation range of the first parameter constitutes a maximum inan intended treatment range of the patient; a memory in which a value ofthe second parameter, for which the variation range of the firstparameter constitutes a maximum, is retained; and control means forchanging the therapeutic value as a function of the first parameterwhile maintaining the previously stored second parameter.
 2. A deviceaccording to claim 1, further comprising switching means for associatinga different second parameter with each different variation range of thefirst parameter.
 3. A device according to claim 2, further comprisingadditional switching means for switching between different variationranges of the first parameter as a function of a third parameter pickedup within the patient's body,wherein the switching and thus theselection of the variation ranges of the first parameter are effected sothat a difference of the values of the first parameter constitutes amaximum in the selected variation range when the second parameter ischanged.
 4. A device according to claim 1, wherein the second parameteris a time window for signal pickup in a cyclic sequence or the locationwithin the body for the derivation of the parameter constituting aninput value.
 5. A cardiac pacemaker including:a circuit for generating atherapeutic value based on a requirement-dependent change of astimulation rate, with a change in electrical conductivity within theheart being a measure, as a first parameter, for physical stress andrequired pumping output of the heart, respectively, and thus an inputvalue for the circuit for the requirement-dependent change of thestimulation rate, with a change in the first parameter being a functionof a second parameter which also constitutes an input value, the circuitfor requirement-dependent change of a stimulation rate comprising:meansfor varying generation of the therapeutic value by varying the secondparameter so that a variation range of the first parameter constitutes amaximum in an intended treatment range of the patient; a memory in whicha value of the second parameter, for which the variation range of thefirst parameter constitutes a maximum, is retained; and control meansfor changing the therapeutic value as a function of the first parameterwhile maintaining the previously stored second parameter; wherein adifference between electrical conductivity at the beginning and end of atime interval within a cardiac cycle constitutes at least indirectly aninput value corresponding to the second parameter for the circuit forthe requirement-dependent change in the stimulation rate, and the timeinterval is utilized for forming the difference, and wherein thedifference of the respective differences between two different stressstates of the patient constitutes a maximum.
 6. A cardiac pacemakeraccording to claim 5, wherein the different stress states of the patientinclude a state of low physical stress corresponding to a rest state,and a range of relatively great physical stress corresponding to aworking state.
 7. A cardiac pacemaker according to a claim 6, whereinabsolute values of the differences are utilized.
 8. A cardiac pacemakeraccording to claim 6, wherein integrals with respect to time of theelectrical conductivities are evaluated.
 9. A cardiac pacemakeraccording to claim 8, wherein the integrals with respect to time of theelectrical conductivities are evaluated according to the Simpsontheorem.
 10. A cardiac pacemaker according to claim 8, wherein gradientchanges in the electrical conductivity having different signs are alsoevaluated.
 11. A cardiac pacemaker according to claim 8, wherein theelectrical conductivity is evaluated only in regions where theelectrical conductivity is monotonous.
 12. A cardiac pacemaker accordingto claim 6, wherein the different stress states of the patient includedifferent stress zones ranges, and wherein the circuit for therequirement-dependent change of the stimulation rate includes switchingmeans for switching between the different stress zone ranges.
 13. Acardiac pacemaker according to claim 12, further comprising an activitysensor, and means for changing the influence of the second parameter onthe first parameter, wherein changing of the influence is effected underthe control of the activity sensor.
 14. A cardiac pacemaker according toclaim 12, wherein the activity sensor simultaneously forms a calibrationsignal.
 15. A cardiac pacemaker according to claim 13, wherein theswitching between zone ranges of different stresses is effected underthe control of the activity sensor.
 16. A cardiac pacemaker according toclaim 13, wherein the switching between zone ranges of differentstresses is effected by an output signal of the circuit for thestress-dependent change of the stimulation rate.
 17. A cardiac pacemakeraccording to claim 16, wherein the cardiac pacemaker is self-adapting inthat, during a stimulation operation, time period regions with smallchanges in increase of electrical conductivity are replaced with oneshaving a greater change in increase of electrical conductivity, wherebythe influence of the second parameter on the first parameter isincreased.